Heretofore it has been conventional to provide certain wheel-mounted cranes with outriggers that could be extended and engaged with the ground to provide fixed, stable support for the crane while it was in operation. Such a crane, when working with its outriggers in place, could not transport a load that it was hoisting, although retraction of the outriggers permitted the unloaded crane to be moved from place to place on its wheels.
A heretofore conventional crawler crane has been capable of transporting a load that it was carrying, since the endless tracks or crawlers on such a crane provided it with a stable base, both when it was moving and when it was standing still. But a crawler crane could not travel on its crawlers for over-the-road transportation from one job site to another; hence, transportation of a crawler crane across any substantial distance required that the machine be loaded onto a flatbed trailer or a railroad car, or that it be disassembled, transported in several loads, and reassembled at a new job site.
Stationary cranes were also available that were intended to be erected at a job site where hoisting operations were to be conducted for a relatively long period of time, and which thereafter had to be disassembled for transportation to a new project at which they were again reassembled.
Each of these types of cranes was relatively specialized, having certain definite advantages and certain definite disadvantages. A contractor who wanted to be able to provide a crane suitable for any type of hoisting operation therefore had to invest in several different cranes, each relatively expensive in itself.
It seldom happens that a contractor has several different jobs simultaneously, each suitable for one of several different cranes. Therefore it has heretofore been necessary for a contractor either to restrict his investment to one or a few cranes and decline jobs for which he was not equipped, or to invest in a variety of cranes only to find that at almost any given time at least one of them was standing idle and thus representing a substantial investment that was yielding no return.
The present invention removes this long-standing dilemma by providing a relatively heavy-duty crane which is substantially more versatile than any heretofore available, so that a contractor or operator can, in effect, equip himself with several different kinds of cranes at a capital cost only slightly higher than was heretofore required for one specialized crane.
It is a more specific object of this invention to provide a convertible crane of modular construction, capable of being altered from one type of crane to another by means of readily interchangeable modules, so that a contractor or crane operator can invest in a set of modules that will provide him with one basic type of crane and, when the need arises, can purchase a few additional modules at relatively modest cost that will enable him to convert to a basically different type of crane.
Another and more specific object of this invention is to provide a convertible crane of modular construction that can be very readily set up as a crawler crane, a stationary crane on a very stable base, or an extra-heavy-lift crane having supplementary counterweight means, and which is further readily convertible to a wheeled transport configuration for over-the-road travel and can be used in that configuration as a truck crane.